1. Field of the Invention
This invention relates an optical element and an optical apparatus having the optical element, and particularly to an optical element having a shape with which distortion occurring to an effective optical portion due to fixing when the optical element is mounted on a casing or the like can be reduced (preferably eliminated), and an optical apparatus having the optical element.
2. Related Background Art
As a method of mounting an optical element on a casing or the like, there has heretofore been fixing by a screw, an adhesive of an ultraviolet curing type or the like, nipping or pressing.
In recent years, with the downsizing of an apparatus, it has often become the case that downsizing is also required of the interval optical element, and the requirement for accuracy has become severer. Due to the downsizing of the optical element, it has become difficult to make a fixing portion large for mounting, while on the other hand, it has become more necessary to reduce the influence of distortion upon an effective optical portion when such a downsized optical element is held and fixed.
So, in International publication No. WO2004/055569A1, disclosed is an optical element comprising an effective optical portion 6 and an optical element holder 1 for fixing and mounting the effective optical portion 6 thereon, and the effective optical portion 6 is provided with one or more slits 8 as distortion stress absorbing means in the end portion thereof in the thickness direction thereof.
Also, the optical element holder 1 is of optical element fixing structure 10 provided with pressing means 3 for pressing at least one of the front surface and rear surface of the effective optical portion 6 outside the slit or slits 8 to thereby fix the effective optical portion 6.
That is, International publication No. WO2004/055569A1, as shown in FIG. 12 of the accompanying drawings, proposes means for providing a slit 8 having thickness equal to or greater than a half of the thickness of the optical element to thereby prevent pressure distortion from propagating to the effective optical portion when the effective optical portion 6 is pressed and fixed to the optical element holder 1.
However, in the region wherein the fixing portion and an effective optical portion are connected together, the influence of stress or distortion due to fixing propagates not a little to the effective optical portion. Accordingly, the region in which the fixing portion and the effective optical portion are connected together should desirably be made small to the utmost. Or it is necessary that the distance from the region in which the fixing portion and the effective optical portion are connected together to the effective optical portion be made long. Also, as in the conventional optical element of International publication No. WO2004/055569A1 mentioned above, when provision is made of a slit equal to or greater than a half of the thickness direction, there arises the problem in strength that a crack occurs from the portion (connection portion) in which the slit is formed and causes rupture.
When the thickness of the portion (connection portion) in which the slit is formed is small, there arises the problem that a crack occurs to thereby cause rupture.
That is, the construction of International publication No. WO2004/055569A1 (FIG. 12) cannot realize a construction for keeping the strength in fixing the effective optical portion by the fixing portion and yet, reducing the influence of distortion upon the effective optical portion.